1. Field of the Invention
The present invention relates to an apparatus for adjusting applied voltage in a display system using a diffractive optical modulator, which can measure the intensity of diffracted light emitted from the diffractive optical modulator, and adjust voltage to be applied to the diffractive optical modulator based on measurement results.
2. Description of the Related Art
Active research into various Flat Panel Displays (FPDs) has been conducted to develop next generation display devices. Among them, generalized FPDs include Liquid Crystal Displays (LCDs) using the electro-optic characteristics of liquid crystal and Plasma Display Panels (PDPs) using gas discharge.
LCDs are disadvantageous in that the viewing angle thereof is narrow, the response speed thereof is slow, and the manufacturing process thereof is complicated because Thin Film Transistors (TFTs) and electrodes must be formed through a semiconductor manufacturing process.
In contrast, PDPs are advantageous in that the manufacturing process thereof is simple, and is therefore suitable for the implementation of a large-sized screen, but are disadvantageous in that the power consumption thereof is high, the discharge and light emission efficiency thereof is low, and the price thereof is high.
New types of display devices, which can solve the disadvantages of the above-described FPDs, have been developed. Recently, there has been proposed a display device that can display images using micro Spatial Light Modulators (SLMs) that are formed for respective pixels using Micro Electromechanical Systems (hereinafter abbreviated as “MEMSs”), which are based on an ultra-micro machining technology.
The SLMs are converters that are configured to modulate incident light into a spatial pattern corresponding to an electrical or optical input. The incident light may be modulated with respect to phase, intensity, polarization or direction. Optical modulation can be achieved using several materials that have several electro-optic or magneto-optic effects, or material that modulates light through surface deformation.
FIG. 1 is a perspective view of a conventional open hole-based diffractive optical modulator.
Referring to the drawing, the conventional open hole-based diffractive optical modulator includes a substrate 101.
The open hole-based diffiactive optical modulator further includes an insulating layer 102 that is formed on the substrate 101.
The open hole-based diffractive optical modulator further includes a lower reflective part 103 that is formed on part of the insulating layer 102 and is configured to reflect incident light that passes through the holes 106aa to 106nb of upper reflective parts 106a to 106n and the spaces between the upper reflective parts 106a to 106n. 
The open hole-based diffractive optical modulator further includes a pair of side support members 104 and 104′ that allow the lower reflective part 103 to be interposed therebetween, and are formed on the surface of the substrate 101 and spaced apart from each other.
The open hole-based diffractive optical modulator further includes a plurality of laminate support plates 105a to 105n that have side portions supported by the pair of side support members 104 and 104′, are spaced apart from the substrate 101, have central portions movable upward and downward, have holes (not shown) corresponding to the holes 106aa to 106nb formed in the upper reflective parts 106a to 106n at the central portions thereof, and constitute an array.
The open hole-based diffractive optical modulator further includes the upper reflective parts 106a to 106n that are respectively formed at the central portions of the laminate support plates 105a to 105n, have the holes 106aa to 106nb at the centers thereof, so that they reflect some incident light and allow the remaining incident light to pass through the holes 106aa to 106nb, and constitute an array.
The open hole-based diffractive optical modulator further includes a plurality of pairs of piezoelectric layers 110a to 110n and 110a′ to 110n′ that are formed over the laminate support plates 106a to 106n, are spaced apart from each other, are placed over the side support members 104 and 104′, and are configured to move the laminate support plates 106a to 106n upward and downward.
In the piezoelectric layers 110a to 110n and 110a′ to 110n′, when voltage is applied to the lower or first electrode layers 110aa to 110na and 110aa′ to 110na, the piezoelectric material layers 110ab to 110nb and 110ab to 110nb′ and the upper or second electrode layers 110ac to 110nc and 110ac′ to 110nc, the central portions of the laminate support plates 105a to 105n move upward and downward due to the contraction and expansion of the piezoelectric material layers 110ab to 110nb and 110ab′ to 110nb′. Accordingly, the upper reflective parts 106a to 106n move upward and downward. For convenience of description, a unit, including each of the laminate support plates 106a to 106n, each of the upper reflective parts 106a to 106n, and each pair of the piezoelectric layers 110a to 110n and 110a′ to 110n′, is referred to as an element.
Meanwhile, when light is incident on the upper reflective parts 106a to 106n of the open hole-based diffractive optical modulator, the upper reflective parts 106a to 106n reflect part of the incident light and allow the remaining part of the incident light to pass through the holes 106aa to 106nb, and the lower reflective part 103 reflects light that has passed through the holes 106aa to 106nb of the upper reflective parts 106a to 106n. 
As a result, the light reflected from the upper reflective parts 106a to 106n and the light reflected from the lower reflective part 103 forms diffracted light having several diffraction coefficients. The intensity of the diffracted light is highest when the difference in height between the upper reflective parts 106a to 106n and the lower reflective part 103 is an odd multiple of λ/4 (where λ is the wavelength of the incident light), and is lowest when the difference in height between the upper reflective parts 106a to 106n and the lower reflective part 103 is an even multiple of λ/4.
FIG. 2 is a partial sectional view of the open hole-based diffractive optical modulator, which is taken along line A-A′ of FIG. 1, and is a sectional view of first upper reflective parts 106a and second upper reflective parts 106b. 
In FIG. 2, when the interval between the upper reflective parts 106a and 106b and the lower reflective part 103 formed on the insulating layer is a first interval
      n    ⁢                  ⁢    λ    2(where λ is the wavelength of incident light and n is an integer), the intensity of light is lowest. Furthermore, when the interval between the upper reflective parts 106a and 106b and the lower reflective part 103 formed on the insulating layer is a second interval
            λ      4        +                  n        ⁢                                  ⁢        λ            2        ,the intensity of light is highest. Meanwhile, in order to exhibit the highest intensity of light, the first upper reflective parts 106a, indicated by solid lines, must be displaced by l1 or L1, and the second upper reflective parts 106b must be displaced by l2 or L2.
However, there may be a case where the upper reflective parts 106a and 106b are located at locations indicated by the dotted lines, not the initial locations indicated by the solid lines, even when voltage is not applied to the piezoelectric layers 110a, 110a′, 110b and 110b′, due to frequent upward and downward movements over time. In this case, in order to obtain the lowest intensity of light or the highest intensity of light, the first upper reflective parts 106a and 106b must be displaced by l1′ or L1′, and the second upper reflective parts 106b and 106b′ must be displaced by l2′ or L2′.
In conclusion, the amount of displacement of the upper reflective parts 106a to 106n for displaying the lowest intensity of light or the highest intensity of light varies. If voltage is applied without considering the variation in the amount of displacement, the intensity of diffracted light, which is expected to be obtained according to the applied voltage, cannot be obtained, thus resulting in the degradation of picture quality characteristics.
Accordingly, the issues that should be considered in the various applications of the above diffractive optical modulator are that there is a possibility that the relationship between an initially set reference applied voltage and the intensity of light may not be maintained any longer when the material/mechanical deformation of the diffractive optical modulator occurs during the operation thereof, and that voltage must be applied in consideration of the variation in the amount of displacement.